The present invention relates to high-strength, high-modulus carbon fibers prepared from a pitch composition as the starting material as well as a pitch composition and precursor fibers therefrom for the preparation of the carbon fibers.
More particularly, the invention relates to high-strength, high-modulus carbon fibers prepared from a specific pitch composition by a process involving the steps of hydrotreatment, high temperature heat treatment, melt spinning, infusibilization and carbonization as well as the pitch composition as the starting material and the precursor fibers obtained therefrom.
As is well known, the carbon fibers currently produced and widely used are classified into two according to the starting material, i.e. the so-called PAN (polyacrylonitrile)-based carbon fibers prepared by the carbonization of polyacrylonitrile fibers and the pitch-based carbon fibers prepared from pitches of coal- or petroleum-origin.
Despite the advantages of the pitch-based carbon fibers due to the inexpensiveness, the PAN-based carbon fibers occupy the major current of the industrial high-performance carbon fibers having high mechanical strength and high modulus suitable for reinforcing various composite materials since the tensile strength of the pitch-based carbon fibers being industrially produced is relatively low and limited to 200 kg/mm.sup.2 or below.
Accordingly, various attempts have been made to develop high-performance carbon fibers starting from inexpensive pitch compositions. Needless to say, the properties of the starting pitch is one of the most important factors for obtaining high-performance pitch-based carbon fibers and several proposals have been made for the preparation of a pitch composition suitable therefor including:
(a) a method in which a specific condensed polycyclic aromatic compound is subjected to a heat treatment or treatment in hydrogen (see, for example, Japanese Patent Publication Nos. 45-28013 and 49-8634);
(b) a method in which a mesophase pitch is obtained by subjecting a tar or pitch of petroleum origin to a first heat treatment in the presence of a Lewis acid catalyst followed by a second heat treatment after removal of the catalyst (see, for example, Japanese Patent Publication No. 53-7533);
(c) a method in which a mesophase pitch having a desired mesophase content is obtained by the heat treatment of a pitch in an atmosphere of a flowing inert gas or under a reduced pressure (see, for example, Japanese Patent Kokai Nos. 53-86717 and 53-86718); and
(d) a method in which an optically isotropic pitch is subjected to a treatment with an organic solvent, e.g. benzene, toluene and heptane, and the insoluble fraction is heated to form neomesophase (see, for example, Japanese Pat. Nos. Kokai 54-160427, 55-58287 and 55-130809).
Unfortunately, the above described methods are not effective enough to give a pitch composition from which high-performance carbon fibers, in particular, in respect of the tensile strength comparable to the PAN-based carbon fibers can be prepared. Therefore, the actual application of carbon fibers prepared from an isotropic pitch is limited to those fields in which no particularly high strength is required for the carbon fibers such as reinforcement in asbestos substitutes. Furthermore, the mesophase pitch produced in some of the above described methods has a problem in the practical manufacturing process due to the extremely high viscosity and poor spinnability thereof giving rise to a difficulty in the process of melt spinning at an economically feasible velocity.
In view of the above described status of the technology for the industrial manufacture of pitch-based carbon fibers, the inventors have continued extensive investigations and previously proposed a method for the preparation of carbon fibers through a specific premesophase pitch which is converted into optically anisotropic mesophase in the course of the carbonization after spinning (see Japanese Pat. No. Kokai 58-18421) followed by the development of a process suitable for the industrial production of such a premesophase pitch.
Further investigations of the inventors have revealed that a very important role is played in the pitch composition for high-performance carbon fibers by the chemical structure and chemical properties of the optically isotropic pitch soluble in quinoline as the starting material for the above mentioned premesophase pitch because the basic structure of the condensed polycyclic aromatic compounds in the pitch is retained as such in the spinning melt deeply influencing the spinnability of the pitch as well as the formation of the internal structures of the fibers in the step of spinning. This discovery has led to a guideline for the completion of the present invention according to which the parameters of particular significance are, for example, the number-average molecular weight of the quinoline-soluble fraction of the pitch, the average molecular weight of the structural units in the quinoline-soluble, optically isotropic pitch, the number of the aromatic rings in the condensed polycyclic structure and the density of the pitch as well as the chemical structure represented by the .sup.13 C-NMR and .sup.1 H-NMR spectra having a specific value of the chemical shifts, the aromaticity index and the H/C ratio in specific ranges.